As MurrayL said, Raphnet-Tech is your best option for adapters.
However, I would honestly recommend picking up an Xbox Series X controller. The build quality is pretty nice, and it's super comfortable in the hand. It has the exact same button layout as the Dreamcast including triggers, just with an extra stick and the bumper buttons, so it's perfect for Dreamcast emulation. Plus, you can use it on all your other games as well.